Thank you for the explanation. I had indeed frozen the player(s) myself before sending the file off to another user for edit using their copy of FMRTE and then their changes reverted when I got the file back. Removing the freezes in both copies of FMRTE solved the issue and the players edited was transferred correctly with the FM file.
As for why multiple people are editing the save game, we're running a "be-a-pro" sim league that updates players' attributes on 16 teams every weekend (thank you for the JSON import function!!). It takes some time so we've needed to divide the workload a bit, hence having multiple people needing to edit the same file.
/Canadice